The invention relates to a pressure sorter for fiber suspensions, with a housing, in which a stationary screen rotationally symmetrical to a screen axis and having a wall thickness of approximately 6 mm to approximately 9 mm is arranged, this screen separating in the housing a supply chamber encircled by the screen from an accepts chamber located outside the screen, wherein an inflow side of the screen faces the supply chamber and the supply chamber and the accepts chamber communicate with one another via through channels located in the screen wall, wherein the through channels, when viewed radially from the supply chamber, are slots approximately parallel to the screen axis, these slots all having the same length as well as an inside or clear slot width measured in the circumferential direction of the screen of approximately 0.1 mm to approximately 0.25 mm and forming in the screen wall a plurality of rows of slots following one another in the circumferential direction of the screen, these rows extending in the circumferential direction of the screen and being arranged in spaced relation to one another in the direction of the screen axis, and wherein the inflow side of the screen includes contours generating-turbulences in the fiber suspension to be treated, as well as a rotor drivable by a motor about the screen axis, an inlet for the fiber suspension to be treated communicating with a first axial end of the supply chamber, an accepts outlet communicating with the accepts chamber and a rejects outlet communicating with a second axial end of the supply chamber, wherein for generating positive and negative pressure pulses in the fiber suspension to be treated the rotor has a plurality of profiled elements arranged in the supply chamber and following one another in the circumferential direction of the rotor, these profiled elements each having a first flank lying in front in the rotational direction as well as approximately parallel to the screen axis for driving the fiber suspension to be treated in the rotational direction of the rotor as well as a second flank lying behind the first flank in a direction opposite to the rotational direction for drawing liquid back from the accepts chamber through the screen and into the supply chamber.
The invention also relates to a process for the preparation of fiber suspensions with the aid of a pressure sorter of the aforementioned type.
In particular, the invention deals with the preparation of such fiber suspensions which are recovered from waste paper.
Pressure sorters of the aforementioned type are generally known. The profiled elements of the rotor can, for example, be freely extending profiled bars, each of which is connected via at least one arm extending approximately radially in relation to the screen axis to a central rotor hub which can rotate about the screen axis; a rotor of this type is disclosed by U.S. Pat. No. 4,410,424 in FIG. 1 as well as U.S. Pat. No. 4,155,841. The rotor can, however, also have a rotationally symmetrical casing about its axis of rotation or about the screen axis with profiled elements attached to its outer circumferential surface which are approximately wedge-shaped in cross section; rotors of this type are disclosed in the PCT application PCT/EP92/01393 published as WO 94/00634 (this corresponds to U.S. patent application Ser. No. 08/351,329) but also in U.S. Pat. Nos. 4,200,537 and 4,855,038. These publications also disclose the fact that the aforementioned, forwardly located first flanks of the profiled elements need not extend exactly parallel to the screen axis or the axis of rotation of the rotor but can also be slightly inclined in relation to this direction.
As already mentioned above, the invention deals with those pressure sorters, in which the through channels in the wall of the screen have the shape of slots which extend at least approximately parallel to the screen axis. Screens having such slot-shaped through channels are shown, for example, in U.S. Pat. Nos. 3,680,696 and 3,726,401.
To prevent the through channels of the screen becoming clogged, during operation of the pressure sorter, by fibers, fiber conglomerations or impurities which are contained in the fiber suspension to be treated, it has proven to be expedient to provide the side of the screen facing the supply chamber of the pressure sorter with such contours that these generate turbulences in the fiber suspension to be treated in cooperation with the profiled elements rotating in the supply chamber. Pressure sorters having a screen, the side of which facing the supply chamber is provided with recesses, into which the through channels open, and/or with elevations or projections for the purpose of generating such turbulences, are described and illustrated in U.S. Pat. Nos. 5,110,456, 4,855,038, 4,276,159 and 4,529,520 as well as in Canadian patent 1,156,609.
The disclosure of all the aforementioned publications is herewith made part of the disclosure of the present application by reference.
The operators of such pressure sorters now require from the producers ever greater throughput capacities of the pressure sorters (volumes of the fiber suspension which can be treated per unit of time). One possibility for increasing the throughput capacity of a pressure sorter is to increase the size of the so-called free, through surface area of the screen (the free, through surface area of the screen is to be understood as the sum of the inside cross-sectional areas of the through channels of all the screen openings of the screen), and a second possibility is to increase the rotational speed of the rotor. However, both measures have so far been subject to limits, namely for the following reasons: During operation of a pressure sorter, the wall of the screen cracks fairly often and therefore becomes unusable, and the occurrence of such cracks is attributed to the fact the screen is constantly exposed to the positive and negative pressure pulses or surges generated by the profiled elements of the rotor in the fiber suspension to be treated. Cracks naturally result in the screen wall due to these pressure pulses all the sooner, the greater the free, through surface area of the screen is, since an increase in the free, through surface area means that the areas of the screen wall remaining between the screen openings become ever smaller. Furthermore, cracks form in the screen wall all the sooner, the thinner the wall thickness of the screen is and the greater or stronger the positive and negative pressure pulses generated by the rotor are, and the strength of these pressure pulses increases with the rotational speed of the rotor. In this connection, it is to be noted that screen wall thicknesses of less than approximately 6 mm are insufficient in most cases and screen wall thicknesses of 10 mm and more lead to material and production costs which are hardly to be justified financially.